This invention relates to ion-beam apparatus and in particular to a system for incorporation in ion-beam equipment to provide ion species selection and optional beam blanking. The invention has particular, although not exclusive, application in the manufacture of microelectronic devices, both for the production of masks and for operations on substrates for the direct production of devices and microcircuits.
The ion sources commonly used in ion-beam equipment may produce a variety of ion species. If the source contains only a single element, then there may be ions with multiple charges and ions with more than one atom; if more than a single element is present in the source, then ions of more than one element may be present in the beam. For many applications in the microfabrication of electronic devices and circuits, and in image formation and surface analysis, it is necessary to use ion beams containing only a single ion species. It is also necessary to provide a means for blanking the ion beam, for turning it off abruptly so that it ceases to impinge on the target.
For lithography applications, the ion beam may be used for resist exposure, for semiconductor doping, and for micromachining. In all these applications it is necessary to select the ion species with the desired penetration characteristics into the target, and to exclude those with other penetration characteristics and those which could contaminate the target. If the beam is to be used for doping, the requirement is for the selection of the electrically active species, whilst for resist exposure and for micromachining it is necessary to select the species with the desired penetration characteristics. In any application it is necessary to exclude ion species with different penetration characteristics and species that may contaminate the target. It is therefore appropriate to consider the design of filters for ion beam lithography instruments.
The ion species selection and beam blanking system of the invention may be employed with a variety of types of ion source. However, it is intended primarily for use with liquid or gaseous field-ionization sources, since they yield a small and intense source of ions which is particularly suitable for equipment in which a finely focused ion probe is required. In such sources, ions are produced by means of a high electric field acting at the apex of a sharply pointed needle in a gaseous environment (see Muller E. W. and Tsong S. S. 1969 "Field ion Microscopy", Elsevier, New York; or Hanson G. R. and Siegel B. M. 1979 J. Vac. Sci. Technol. 16 1875-1878) or wetted with a liquid-metal film (see Clampitt R., Aitken K. L. and Jefferies D. K. 1975 J. Vac. Sci. Technol. 12, 1208 or Prewett P. D., Jefferies D. K. and Cockhill T. D. 1981 Rev. Sci. Instrum. 52, 562-566), or at the apex of a capillary tube containing the liquid to be ionized (see Mahoney J. F. et al 1969 J. Appl. Phys. 40, 5101-5106). In the case of the liquid-metal field-ionization sources, the liquid may be either a pure metal or an alloy.
One known type of system which has been adopted for ion beam apparatus (see Wang, V. et al J. Vac. Sci. Technol. 19 1158) is the Wien filter, in which an ion beam passes through superimposed electric and magnetic fields which are nominally perpendicular and proportional. Provided the fields are accurately matched, the electrical axis of the filter is straight; however, there is no means for eliminating neutral atoms or droplets. In order to produce the matched, superimposed fields, it is necessary to have relatively large gaps between the magnet pole pieces (to accommodate the electrodes) and between the electrodes (to accommodate the pole pieces). Thus the fields are inevitably fairly weak, whilst requiring high magnetic excitations and voltages. This is acceptable for the apparatus described by Wang V. et al 1981 J. Vac. Sci. Technol. 19 1158, since the filter is introduced into a low voltage section of the column rather than at the full beam voltage of 150 kV. A further limitation is that the filter would be dispersive with respect to energy distribution within a single ion species.